The present invention relates to an isolation valve, designed in particular for equipping a liquefied gas storage tank.
Conventionally, a gas storage tank comprises isolation means allowing any flow of gas out of the tank to be prevented. To this end, it is known to equip the tank with a movable safety valve mounted in a gas extraction pipe and blocked by a fusible pin. In normal operation, the safety valve is held by the pin in a position wherein it does not impede the flow of the liquefied gas. In the event of fire, the pin yields and frees the safety valve, which blocks the flow of gas. In practice, such a safety valve only makes it possible to limit the flow of gas, but does not afford a total seal. A valve is therefore added downstream of the safety valve, said valve providing the sealing function even if it is subjected to high temperatures in the event of a fire. This valve has the following disadvantages.
First of all, a valve is designed to control the flow of fluid through a duct and not to provide an isolation function. Consequently, such a valve is generally not perfectly sealed. Moreover, actuation of such a valve, even if it is motorized, is relatively slow. Indeed, the valve is actuated by rotation of a shaft, and transition from the fully open position to the fully closed position requires several turns.
It is known, moreover, to use for different applications, valves offering a good seal, such as in particular valves of the “double block and bleed” or DBB type, described in particular in documents EP 1 618 325 and WO 2010/131039. A valve of this type conventionally comprises a body including a fluid flow channel, designed to be plugged by upstream and downstream spherical plugs, operable independently of one another between positions for fluid flow through said channel and positions plugging the channel. The body is also equipped with bleeding means leading into the aforementioned channel, between the upstream and downstream plugs.
The use of two means of isolation in series, consisting of the upstream and downstream plugs, makes it possible to guarantee that no fluid escapes, even in the event of failure of one of the isolation means. The bleeding means make it possible to check the proper sealing of the plugs. To this end, it is sufficient to close the plugs, to bleed the part of the channel located between the two plugs a first time, then checking with a second bleed that at the end of a predetermined period no fluid has escaped downstream of the upstream plug.
The “double block and bleed” type valve also comprises, for each upstream and downstream plug, an upstream seat and a downstream seat, associated with elastic return means so as to come to bear sealingly against the spherical plugs around the fluid flow channel. Such a valve comprises several dead volumes, that is to say confined volumes capable of capturing fluid.
In the case where both plugs are in the plugging position, a first dead volume is formed in the fluid flow channel, between the upstream and downstream plugs. A second dead volume is formed by a volume external to the aforementioned channel, located between the outer wall of the upstream plug and the body. The second dead volume also comprises the internal volume of the upstream plug which, in the plugging position of the upstream plug, communicates with the aforementioned external volume.
Finally, a third dead volume is formed by a volume external to the aforementioned channel, located between the outer wall of the downstream plug and the body. The third dead volume also comprises the internal volume of the downstream plug which, in the plugging position of the downstream plug, communicates with the aforementioned external volume.
In the event of fire, the gas caught inside these dead spaces expands and can cause the cracking, even the explosion of the valve body, causing the complete destruction of the valve. The gas can then escape through the cracks that are formed, thus supplying fuel to the fire. In order to allow the escape of the gas contained in the second and third dead volumes, the seats of the upstream and downstream plugs can be of the “simple piston effect” or SPE type. In this case, the seats are designed to detach from the corresponding plugs in the event of overpressure in the dead volumes.
Such a design has the following disadvantages. First of all, gas can escape to the outside, downstream, due to detachment of the downstream seat from the downstream plug. Even if only a limited volume of gas is involved (corresponding to the third dead volume), this gas nevertheless supplies fuel to the fire. Furthermore, the seats do not allow to evacuate the gas contained in the first dead volume, between the upstream and downstream plugs, the expansion of this gas which may cause, as indicated above, cracking or explosion of the valve body.
The invention has in particular the aim of providing a simple, effective and economical solution to this problem.